Most of our planet comprises seawater and with a salt content of 3.5% this needs to be reduced to <0.05% (or less) to make it drinkable. Some of the older processes used distillation which requires about 10 kWh of energy per m3 of seawater. The seawater is heated up and the resulting water vapour is condensed into drinkable form.
The other popular approach is reverse osmosis with semi-permeable membranes which sieve out the sodium and chloride ions (constituents of salt) and only pass the fresh water through. The pressure to accomplish this ‘costs’ about 4 kWh per m3 of salt water; considerably less. Careful maintenance of the molecular screens is required here to prevent fouling from pollutants and sea creatures.
Another simple approach (remember the KISS principle) is solar desalination where the sun is used to heat a volume of water, which causes evaporation. The fresh water recovered drips into a collection system. The limitation here is that there is a theoretical maximum amount of water that can be evaporated by the sun in a given area.
Channels with membranes
Another new process called electrodialysis is very clever (well, I think so). The seawater is pumped through a series of channels with membranes which are dedicated to passing either Sodium or Chloride ions when the appropriate voltage is applied. The fresher water is repeatedly passed through this process until its salt concentration is reduced to below 1%. An ion-exchange resin is used to bring the salt concentration below 0.5%. The energy required to do this is about 1.8 kWh per m3. Dramatically lower than distillation discussed above.
Many see the cost of these desalination plants (and use of high levels of energy) as a great issue. And regard it as cheaper to pump water out of an existing water acquifer underground. However, the problem is that the acquifer is non-renewable.
Renewable Energy isn’t always powering these desalination plants
Many marketing types are claiming that renewable energy is powering the desalination plants; hence there is no carbon footprint. However, this is not strictly true. A reverse osmosis desalination plant works continuously and power consumption is constant. Whereas renewable energy sources are intermittent and storage of the amount of energy required by a reverse osmosis plant virtually impossible. Not to say that increasing the size of renewable energy capacity isn’t admirable. It is. But we have to recognise the limitations and dynamics of power grids, power consumers and generation.
Obviously other solutions are to reduce the consumption of fresh water by 50% with toilet flushing etc.
Other suggestions are to use waste heat from industrial processes, which would otherwise be discarded (hence doesn’t directly cost) to perform distillation.
Another (perhaps more rural) idea is to create a rainwater capture facility which will capture more drinking water at a lower cost. I know my folks used one for many years, capturing the rainwater from their roof and using the water throughout the year (although one had to carefully watch out for dead rats and other detritus in the tank).
What does this all mean to us?
- As we are repeatedly told – fresh water is scarce and needs to be treated with care. Try and use less
- When examining a desalination system; examine all the options with great care with reference to renewable energy – there are new systems developing all the time
As anyone who has swallowed salt water knows: ‘Revenge has no more quenching effect on emotions than salt water has on thirst’ (from Walter Weckler).
Thanks to the Economist and Dr A. Jagadeesh Nellore and Mr Ah Beng for useful information on desalination processes.
Yours in engineering learning